The main fields of application of a conventional radar technique are defense and meteorology. The production of radar apparatuses in a small quantity is sufficient for those fields, and the achievement of requested performance is demanded even at the expense of cost. For an on-vehicle radar, which is a new field of application of the radar technique, however, the scale of mass production can be from tens of thousands to hundreds of thousands in number depending on the degree of market saturation, even millions in some cases. For the diffusion of the on-vehicle radars, production cost thereof is required to be reduced.
In order to supply a low-cost radar apparatus satisfying such requirements, besides a low-load signal processing method which replaces a signal processing method requiring a high-performance processing circuit as employed in the conventional radar technique, an antenna configuration is required to be simplified and to lower transmission power.
On the other hand, the on-vehicle radar has an object of detecting an obstacle in driving to avoid danger, thereby enhancing the driving safety. In some cases, a crucial process involving a human life has to be executed with good accuracy. Therefore, although it is true that the cost is required to be reduced, the performance of detecting a target is not allowed to be sacrificed. More specifically, a target present in a relatively wide angle range, in the range of 0 m to about 200 m in terms of the relative distance, is required to be detected within several milliseconds with a resolution of about 1 m.
Conventionally known radar systems include a pulse system, a pulse compression system (spread spectrum radar), an FMCW system, and a dual-frequency CW system. In the pulse system or pulse compression system radar apparatus, for example, in order to obtain a resolution of 1 m, a broadband over 150 MHz is required. In the case of the pulse compression radar, in particular, a computation load for correlation processing is large, requiring signal processing at a high speed. As a result, those systems are disadvantageous in terms of production cost reduction.
On the other hand, in the systems such as the frequency modulated continuous wave (FMCW) system, the dual-frequency CW system, and a multi-frequency CW system, it is known that a desired range resolution is obtained through signal processing at relatively low speed. Therefore, because those systems have an advantage in that the demands for reduction in production cost and increase in resolution can be satisfied at the same time, they are expected to be used for the on-vehicle radar.
In reality, however, those systems using a continuous wave (CW) as a transmitted wave (hereinafter, the radar systems using a continuous wave are collectively referred to as “CW system”) has significant tasks such as ensuring isolation between transmission and reception and taking countermeasures against unnecessary reflected wave with a small propagation loss from a short range (nearby clutter). Therefore, in order to cope with such problems, systems including a frequency modulated interrupted continuous wave (FMICW) system in which a continuous transmitted wave is pulsed are known.
Further, the on-vehicle radar is required to detect a target present in a wide angle range, such as vehicles running ahead in a plurality of lanes. In order to satisfy the requirement, an antenna system for mechanically driving an aperture antenna, a multi-beam antenna system, a phased-array system including an array antenna, a digital beam forming (DBF) system, and the like are known.
Among the above systems, the mechanical driving systems require a long observation time for observing a reflection source in a wide coverage. Therefore, it is difficult to enhance response performance. The multi-beam antenna system for simultaneously emitting transmitted beams in multiple directions and the phased-array system including an array antenna which uses electrical phase control to change a beam scanning direction within a short period of time have no problem in response performance. However, those systems are not suitable for mass production because the mechanism of the antenna is too complicated.
On the other hand, the DBF system realizes beam formation by digital signal processing and is advantageous in excellent process adaptability, scalability, and high-resolution. In particular, the on-vehicle radar is requested to detect an object present in a wide coverage within a short period of time (ideally, within several milliseconds). Therefore, the combination of a beam having a large beam width referred to as wide-angle beam or fan beam and the DBF system is advantageous as a system for the on-vehicle radar.
The wide-angle beam has such a wide beam width that a single pulse illuminates the entire requested coverage. With the combination of the wide-angle beam and the DBF method, after a beam having such a wide beam width that the requested coverage can be covered at one time is transmitted, an echo of the transmitted beam is captured by an array antenna. Thereafter, according to the DBF method, an output signal from each of element antennas of the array antenna which has captured the beam is subjected to digital signal processing to form a beam in an arbitrary direction (see, for example, Patent Documents 1 and 2).
In this manner, a mechanism of controlling a beam direction is simplified while the overhead of beam scanning is kept minimum. At the same time, a target in the requested coverage can be detected within a short period of time. In addition, the combination of the wide-angle beam and the DBF method can be further combined with the radar principle such as the FMCW system, the dual-frequency CW system, and the multiple-frequency CW system. Further, the pulsation of the wide-angle transmitted beam solves most of the problems caused in providing a practical on-vehicle radar.
Patent Document 1: U.S. Pat. No. 5,497,161 A
Patent Document 2: JP 3622565 B2